This invention relates to ceramic capacitors and in particular, but not exclusively, to multilayer ceramic capacitors and dielectric compositions for use therein.
A multilayer ceramic capacitor basically comprises a stack consisting of a plurality of dielectric members formed of a ceramic material, with electrodes positioned between the members. The electrodes may be screen-printed onto the ceramic material, in the unfired state thereof, using conductive inks. A stack of screen-printed dielectric members is assembled, pressed together, cut into individual components, if appropriate, and fired until sintering occurs, in order to ensure non-porosity. The internal electrodes may be of rectangular form and cover the whole or part of the area of the adjacent dielectric layers. The internal electrodes in successive layers may be sideways stepped relative to one another or have elongate portions which cross one another, as described in our G.B. Application No. 7841677 (Serial No. 2032689B) (A. Oliver-G. Mills 1-1).
With the conventionally employed dielectrics the capacitors must be fired at temperatures of the order of 1200.degree.-1400.degree. C., which means that the internal electrodes must be of a suitable material to withstand such temperatures and that, therefore, expensive noble metals, such as platinum or palladium must be used. However, if the firing temperatures can be reduced, by a suitable choice of the dielectric, then internal electrodes with a high silver content (50-100% silver) could be used, thus reducing costs for materials and manufacture. In our co-pending G.B. Application No. 8120605 (Serial No. 2107300A) (J. M. Wheeler 1) there is disclosed a dielectric composition which can be fired at a temperature between 950.degree. C. and 1100.degree. C. and can thus be used with high silver content internal electrodes. These low firing temperature dielectrics comprise lead magnesium niobate (PbMg.sub.1/2 Nb.sub.1/2 O.sub.3) with one or more of the following, namely lead titanate, lead stannate, lead zirconate, and some of these dielectric compositions have dielectric constants in the range 7,500-10,000, which makes them particularly suitable for multilayer ceramic capacitors. The conventionally employed ceramics (U.S. coding Z5U) are not compatible with high silver content electrodes and usually have dielectric constants lower than 7,500-10,000. The electronics industry, generally, requires smaller component, and smaller and cheaper capacitors can be obtained by producing dielectrics which are compatible with high silver content electrodes and even higher dielectric constants than the 7,500-10,000 range mentioned above with reference to our co-pending Application No. 8120605.
In another co-pending G.B. Application No. 8317265 (Serial No. 2126575)(J. M. Wheeler 2X) there is disclosed a dielectric composition comprising lead magnesium niobate (PbMg.sub.1/2 Nb.sub.1/2 O.sub.3), non-stoichiometric lead iron niobate and one or more oxide additives, which may be chosen from silica, manganese dioxide, ceric oxide, lanthanum oxide, zinc oxide, alumina, tungsten oxide, nickel oxide, cobalt oxide and cuprous oxide. If, for example, three or more oxide additives are chosen from the first eight of the ten mentioned above compositions having firing temperatures between 980.degree. C. and 1075.degree. C. may be obtained, the dielectric constants after firing being in the range 10,600 to 16,800, making them particularly suitable for small multilayer ceramic capacitors with high silver content electrodes. Additionally the dielectric composition may comprise lead titanate (PbTiO.sub.3).
In a further co-pending G.B. Application No. 8405677 (Serial No. 2137187)(J. M. Wheeler-D. A. Jackson 3-1X), there is disclosed a dielectric composition comprising lead magnesium niobate and lead zinc niobate. This dielectric composition may also include one or more oxide additives chosen from silica, maganese dioxide, zinc oxide, nickel oxide, alumina, ceric oxide, lanthanum oxide, tungsten oxide, gallium oxide, titanium dioxide and lead oxide. One or more of the following may also be added to the basic composition, bismuth stannate, bismuth titanate, lead stannate, lead zirconate and lead titanate with or without an oxide additive. Such compositions fire at temperatures between 980.degree. C. and 1075.degree. C. and have dielectric constants in the range 9,000 to 16,300 with Z5U temperature dependence characteristics and low tan .delta.(%) (dielectric loss).
Lead magnesium niobate is conventionally understood to mean PbMg.sub.1/3 Nb.sub.2/3 O.sub.3, however the lead magnesium niobate which we have used in all of the dielectric compositions referred to above is not the conventional variety and has been generally referred to as PbMg.sub.1/2 Nb.sub.1/2 O.sub.3. The material we employed for the results quoted in the above mentioned applications is in fact PbMg.sub.0.443 Nb.sub.0.5001 O.sub.3 and since that approximates to PbMg.sub.1/2 Nb.sub.1/2 O.sub.3 the latter expression has generally been used to distinguish from the conventinal PbMg.sub.1/3 Nb.sub.2/3 O.sub.3. Preferably, however, for our purposes the magnesium was in the range 0.35 to 0.5 and the niobium was in the range 0.4 to 0.8 i.e. PbMg.sub.0.35 to 0.5 Nb.sub.0.4 to 0.6 O.sub.3, thus the lead magnesium niobate with which we were concerned was, generally, non-stoichiometric. The expression lead zinc niobate is conventionally understood to mean PbZn.sub.1/3 Nb.sub.2/3 O.sub.3, however non-stoichiometric versions are also possible and that used in co-pending Application No. 8405677 (Wheeler 3-1X) was defined as PbZn.sub.0.3 to 0.5 Nb.sub.0.6 to 0.7 O.sub.3.
It is an object of the present invention to provide alternative dielectric compositions which can be used with high silver content electrodes and have higher dielectric constants than the Z5U compositions.